1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method. In the apparatus, an electrostatic latent image is formed on an image carrier and toner moves to a surface of the image carrier from a toner carrier which carries the toner to thereby visualize the electrostatic latent image and form a toner image.
2. Description of the Related Art
Known as image forming apparatuses, such as copier machines, printers and facsimile machines, to which electrophotographic techniques are applied are two types: those apparatuses of the contact developing type according to which an image carrier and a toner carrier are held abutting on each other; and those apparatuses of the non-contact developing type according to which an image carrier and a toner carrier are held away from each other. Of these, in an image forming apparatus of the contact developing type, a toner carrier is applied a developing bias with a direct current voltage or a voltage which is obtained by superimposing an alternating current voltage upon a direct current voltage. When toner carried by a surface of the toner carrier contacts an electrostatic latent image which is formed on an image carrier, the toner partially moves toward the image carrier in accordance with a surface potential of the electrostatic latent image, and a toner image is consequently formed.
Meanwhile, in an image forming apparatus of the non-contact developing type, an alternating voltage serving as a developing bias is applied upon a toner carrier. This causes an alternating field develop in a gap between the toner carrier and an image carrier. Toner transfers onto the electrostatic latent image owing to the function of the alternating field, and a toner image is consequently formed.
In such an image forming apparatus, an image density of a toner image may cyclically change because of variable factors related to a structure of the apparatus. The variable factors may include eccentricity, deformation, a scratch on a surface and the like of a toner carrier or an image carrier, for instance. Further, in an image forming apparatus in which a surface of an image carrier is formed by a photosensitive member and this surface is exposed with a light beam so that an electrostatic latent image is formed. An image density cyclically changes in some cases due to a variation in sensitivity of the photosensitive member within the surface of the image carrier, a change in temperature of the photosensitive member, etc.
Hence, a density of a toner image formed as a patch image, too, changes not only because of settings of density control factors but also in accordance with the density changes described above. When an influence of such a density change is contained in a value which is detected as a patch image density, it is not possible to correctly grasp a correlation between the density control factors and an image density. This further makes it difficult to set the density control factors to appropriate values even despite optimization of the density control factors based on patch image densities.
In a conventional image forming apparatus, density control factors are set based on a density of a patch image without sufficiently considering the influence of density changes attributed to a structure of the apparatus over a patch image density. This may lead to a consequence that an image is formed under an image forming condition which is not an originally intended optimal condition. This may sometimes prevent formation of a toner image which has a sufficient image quality.